If possible, could you check the following please on the GNC420W:

1. Press and hold CLR until the default NAV page displays

2. Turn the large right knob to select AUX page group

3. Turn the small right knob to select SETUP 2

4. Select COMM configuration

5. Check if you can change from 25 to 8,33khz
(Note: for US, 25khz must be set).

Thanks!

Best regards,
Patrick